A power amplifier used in a wireless communication system is required to have linearity and high efficiency. Especially with an explosive growth of the portable terminal market and the associated infrastructure installation in recent years, a multi-level digitally modulated communication system signal having a mean value of signal amplitudes and a maximum amplitude which are significantly different from each other is often handled. When such signal is amplified by a conventional power amplifier, the amplifier is set to such operation point that the signal may be amplified to the maximum amplitude without distorting the signal for operation. Therefore, there is little time when the amplifier is operating near a saturation output at which relatively high efficiency may be maintained, and in general, the amplifier has had low efficiency.
On the other hand, there is a strong demand for improved efficiency in the market, and an attempt to construct an amplifier having low distortion and high efficiency by a combination of a method of amplifying a signal with high efficiency and a technology of reducing and compensating for the distortion of the signal has been attracting attention.
In order to solve the above-mentioned problem, various technologies of increasing the efficiency of the amplifier while maintaining the linearity have been provided. One example thereof is a Doherty amplifier. The Doherty amplifier includes an amplifier (hereinafter sometimes referred to as carrier amplifier) for performing an operation of amplifying a signal at all times, and an amplifier (hereinafter sometimes referred to as peak amplifier), which is called a peak amplifier or an auxiliary amplifier, for operating only at the time of high power output, to divide an input signal to the carrier amplifier side and the peak amplifier side and combine outputs of the carrier amplifier and the peak amplifier to be output.
A basic configuration of such Doherty amplifier is disclosed in Non Patent Literature 1, Patent Literatures 1 and 2, and the like. The Doherty amplifier includes an amplifier for operating in the vicinity of the saturated output power while maintaining saturation, to thereby realize higher efficiency than a general class A or class AB amplifier even when the output is backed off from the saturated power.
As the carrier amplifier, a class AB or class B biased amplifier is generally used. On the other hand, as the peak amplifier, a class C or class BC biased amplifier is generally used so as to operate only when the instantaneous signal power is high output.
In addition, in order to construct a Doherty amplifier which is further improved in efficiency, an amplifier controlled by a harmonic matching circuit, such as a class E or class F amplifier, which is devised not only in the fundamental matching circuit but also in the harmonic matching circuit, has been adapted. In this case, especially in the second harmonic frequency band, it is often preferred that a load of the amplifier be a short-circuit or near short-circuit impedance for improving the amplifier efficiency.
Patent Literature 3 discloses a method of constructing an even harmonic matching circuit and an odd harmonic matching circuit of the carrier amplifier and the peak amplifier of the Doherty amplifier.
In addition, as illustrated in FIG. 1, the Doherty amplifier disclosed in each of Patent Literatures 4 and 5 includes a quarter-wave impedance transformer as an impedance transformer 120 after combining the signals on the output side. The impedance transformer 120 performs impedance transformation so that an impedance looking into the load side from the signal combining point of a carrier amplifier 114 and a peak amplifier 115 becomes (½)×Zo. Here, Zo is a characteristic impedance of the load, and generally, 50Ω is selected therefor.
Further, an example of a more specific method of realizing the combiner on the output side and the impedance transformer with respect to the load is disclosed in FIG. 6 of Patent Literature 6. In the configuration of Patent Literature 6, in order to further improve the efficiency of the Doherty amplifier, in constructing a second harmonic matching circuit, which is effective in attaining high efficiency especially in the harmonic frequency band, the load impedance looking into the load side from the signal combining point is subjected to impedance transformation into (½)×Zo for the fundamental wave. However, with the configuration of Patent Literature 6, the length of the impedance transformer corresponds to a half wavelength in the second harmonic frequency band, which is equivalent to no impedance transformation so that the impedance looking into the load side from the signal combining point at the second harmonic frequency remains Zo.
In constructing the amplifier matching circuit for further improving the efficiency, it is often preferred that, as disclosed in Patent Literature 3, the load of the amplifier be a short-circuit or near short-circuit impedance for improving the amplifier efficiency especially in the second harmonic frequency band.
In view of the above, with the configuration of the conventional Doherty amplifier, there is a need to construct a matching circuit for transforming, at the output terminal of the amplifier, the impedance Zo looking into the load from the signal combining point to the short-circuit or near substantially short-circuit impedance, for example, Zi (<<Zo), in the second harmonic frequency band, and hence a need for a high impedance transformation ratio.
The resulting high impedance transformation ratio leads to the problem in that the frequency band at which the desired short-circuit or near substantially short-circuit impedance may be obtained in the second harmonic frequency band at the output terminal of the amplifier is narrowed, which is disadvantageous in increasing the bandwidth. For example, Patent Literature 7 refers in paragraph [0006] to the increased loss and decreased frequency bandwidth when the matching circuit having high impedance transformation ratio is inserted in the Doherty amplifier.